1. Field of the Invention
The present invention relates to a liquid crystal display device which uses a liquid crystal having spontaneous polarization, and, more particularly, a liquid crystal display device with less display burning. The invention also relates to a method of evaluating and/or predicting the degree of display burning of a liquid crystal display device which uses a liquid crystal having spontaneous polarization.
2. Description of the Related Art
Development is being actively made on liquid crystal display devices that use a ferroelectric liquid crystal and an antiferroelectric liquid crystal which are excellent in the fast response characteristic and the angle of visibility in place of liquid crystal display devices that use a conventionally popular nematic liquid crystal. The liquid crystal display device that uses an antiferroelectric liquid crystal is advantageous over the liquid crystal display device that uses a ferroelectric liquid crystal in easier alignment control of the liquid crystal molecules and higher shock absorption. Therefore, active studies have been made on the former liquid crystal display device.
Liquid crystal display devices which use a ferroelectric liquid crystal or an antiferroelectric liquid crystal have such a difficulty that their transmittances cannot be controlled arbitrarily, thus making it hard to provide gradation display. Recently, antiferroelectric liquid crystal display devices which use an antiferroelectric liquid crystal and can accomplish gradation display have been proposed as disclosed in U.S. Pat. Nos. 5,631,752, 5,895,108 and Japanese Unexamined Patent Publication (KOKAI) No. 64056/1995. As the antiferroelectric liquid crystals disclosed in those publications have a wide voltage range where an antiferroelectric-ferroelectric phase transition precursor phenomenon occurs with respect to the applied voltage, they have a number of intermediate optical states in that range. A liquid crystal display device which is capable of achieving gradation display can be provided by using such a liquid crystal and controlling the applied voltage in such a range where the liquid crystal molecules take intermediate optical states.
In the liquid crystal display devices that use a ferroelectric liquid crystal or an antiferroelectric liquid crystal, when a positive or negative voltage is applied to the liquid crystal for a long period of time, the applied voltage causes an ionic impurity present in the liquid crystal to gather in the vicinity of the electrodes due to the spontaneous polarization of the liquid crystal. The charges that are originated from the ionic impurity interact with the spontaneous polarization of the liquid crystal molecules, so that the alignment state of the liquid crystal molecules is fixed. As a result, even if application of the voltage is stopped, the liquid crystal display devices would have so-called display burning which dimly shows the previously displayed image.
At the stage of designing ferroelectric liquid crystal display devices or antiferroelectric liquid crystal display devices, it has been difficult to predict the aforementioned display burning of the liquid crystal display devices. There is no way but to actually design and make protocols of liquid crystal display devices and take real measurements on the degree of display burning empirically or the like. This takes a considerable time and cost in designing and manufacturing display devices.
Accordingly, it is an object of the present invention to provide an antiferroelectric liquid crystal display device with suppressed display burning.
It is another object of the invention to provide a display-quality evaluation method for a liquid crystal display device, which can evaluate the degree of display burning without actually measuring the degree of display burning.
To achieve the first object, according to the first aspect of the invention, there is provided a liquid crystal display device comprising:
a pair of substrates arranged opposite to each other;
electrodes arranged on opposing surfaces of the pair of substrates and facing each other; and
a liquid crystal provided between the substrates and having spontaneous polarization and a physical property such that when a voltage of one polarity and a sufficiently large level is applied between the electrodes, liquid crystal molecules are aligned in a first direction, when a voltage, of a polarity opposite to the one polarity and a sufficiently large level is applied between the substrates the liquid crystal molecules are aligned in a second direction, and when a voltage lying between the voltage of the one polarity for aligning the liquid crystal molecules in the first direction and the voltage of the other polarity for aligning the liquid crystal molecules in the second direction is applied between the electrodes, a director is aligned in an arbitrary direction corresponding to the applied voltage and in a cone angle formed by the first direction and the second direction, the physical property satisfying a following equation:
xcex5(xcex82/Ps2)xe2x89xa68xe2x80x83xe2x80x83(1)
where xcex5[F/m] is a permittivity of the liquid crystal located between the substrates, xcex8[o] is a tilt angle defined by xc2xd of the cone angle of the liquid crystal, and Ps[nC/cm2] is spontaneous polarization of the liquid crystal molecules of the liquid crystal.
The liquid crystal display device according to the first aspect of the invention is constructed in such a manner that the value of a normalized permittivity xcex5s[=xcex5(xcex82/Ps2)] which is acquired by normalizing the permittivity xcex5 with the square of the tilt angle xcex8 and the square of the spontaneous polarization Ps meets the equation 1. Although the liquid crystal display device uses the liquid crystal having spontaneous polarization that is likely to cause display burning as compared with the conventional TN liquid crystal or the like, therefore, the occurrence of display burning can be restrained. It is therefore possible to provide high-quality gradation display which is less influenced by display burning while making good use of the merits of the liquid crystal having spontaneous polarization, such as an excellent response characteristic.
It is preferable that the normalized permittivity xcex5s be 5 or lower. Further, the equation 1 can be applied to both a ferroelectric liquid crystal and an antiferroelectric liquid crystal, and a liquid crystal display device which uses either type of liquid crystal can effectively suppress display burning.
To achieve the first object, according to the second aspect of the invention, there is provided a liquid crystal display device comprising:
a pair of substrates arranged opposite to each other;
electrodes arranged on opposing surfaces of the pair of substrates and facing each other; and
a liquid crystal provided between the substrates and having spontaneous polarization and a physical property such that when a voltage of one polarity and a sufficiently large level is applied between the electrodes, liquid crystal molecules are aligned in a first direction, when a voltage of a polarity opposite to the one polarity and a sufficiently large level is applied between the substrates, the liquid crystal molecules are aligned in a second direction, and when a voltage lying between the voltage of the one polarity for aligning the liquid crystal molecules in the first direction and the voltage of the other polarity for aligning the liquid crystal molecules in the second direction is applied between the electrodes, a director is aligned in an arbitrary direction corresponding to the applied voltage and in a cone angle formed by the first direction and the second direction, the physical property satisfying a following equation:
C(xcex82/Ps2)xe2x89xa60.8xe2x80x83xe2x80x83(2)
where C[F/cm2] is a capacitance of the liquid crystal display device per unit area, xcex8[o] is a tilt angle defined by xc2xd of the cone angle of the liquid crystal, and Ps[nC/cm2] is spontaneous polarization of the liquid crystal molecules of the liquid crystal.
This liquid crystal display device is constructed in such a manner that as apparent from the equation 2, the value of a normalized capacitance [C(xcex82/Ps2)] which is acquired by normalizing the capacitance C with the square of the tilt angle xcex8 and the square of the spontaneous polarization Ps becomes equal to or smaller than 0.8. Although the liquid crystal display device uses the liquid crystal having spontaneous polarization, therefore, display burning is unlikely to occur. This makes it possible to provide high-quality gradation display which is less influenced by display burning while making good use of the merits of the liquid crystal having spontaneous polarization, such as an excellent response characteristic.
To achieve the second object, according to the third aspect of the invention, there is provided a method of evaluating display burning of a liquid crystal display device comprising a pair of substrates arranged opposite to each other, electrodes arranged on opposing surfaces of the pair of substrates and facing each other, and a liquid crystal provided between the substrates and having spontaneous polarization and a physical property such that when a voltage of one polarity and a sufficiently large level is applied between the electrodes, liquid crystal molecules are aligned in a first direction, when a voltage of a polarity opposite to the one polarity and a sufficiently large level is applied between the electrodes, the liquid crystal molecules are aligned in a second direction, and when a voltage lying between the voltage of the one polarity for aligning the liquid crystal molecules in the first direction and the voltage of the other polarity for aligning the liquid crystal molecules in the second direction is applied between the electrodes, a director is aligned in an arbitrary direction corresponding to the applied voltage and in a cone angle formed by the first direction and the second direction. The method comprises a property evaluation step of acquiring a physical property including a permittivity of the liquid crystal located between the substrates, a tilt angle defined by xc2xd of the cone angle of the liquid crystal and spontaneous polarization of the liquid crystal molecules of the liquid crystal; and a determination step of determining a degree of display burning based on a normalized permittivity xcex5s defined by a following equation 3:
xcex5s=xcex5(xcex82/Ps2)xe2x80x83xe2x80x83(3)
where xcex5[F/m] is the permittivity of the liquid crystal located between the substrates, xcex8[o] is the tilt angle defined by xc2xd of the cone angle of the liquid crystal, and Ps[nC/cm2] is the spontaneous polarization of the liquid crystal molecules of the liquid crystal.
A liquid crystal display device which is to be evaluated by the present invention comprises a liquid crystal having spontaneous polarization, such as a ferroelectric liquid crystal or an antiferroelectric liquid crystal, is likely to suffer display burning and has a gradation display capability. As apparent from the equation 3, the possible display burning of such a liquid crystal display device can be evaluated objectively by using the value of the normalized permittivity xcex5s[=xcex5(xcex82/Ps2)] as an index, so that the degree of display burning of the liquid crystal display device can be predicted without actually measuring the display burning of the liquid crystal display device. This contributes to reducing the time and cost of producing the liquid crystal display device.
The display burning of a liquid crystal display device is allowable within the range where the normalized permittivity xcex5s is equal to or smaller than 8 or more preferably is equal to or smaller than 5 as indicated by the equation 1. Comparing the normalized permittivity xcex5s with those values can therefore make it possible to determine whether the display quality is acceptable or not. The equation 1 is applicable to both a ferroelectric liquid crystal and an antiferroelectric liquid crystal.
To achieve the second object, according to the fourth aspect of the invention, there is provided a method of evaluating display burning of a liquid crystal display device comprising a pair of substrates arranged opposite to each other, electrodes arranged on opposing surfaces of the pair of substrates and facing each other, and a liquid crystal provided between the substrates and having spontaneous polarization and a physical property such that when a voltage of one polarity and a sufficiently large level is applied between the electrodes, liquid crystal molecules are aligned in a first direction, when a voltage of a polarity opposite to the one polarity and a sufficiently large level is applied between the electrodes, the liquid crystal molecules are aligned in a second direction, and when a voltage lying between the voltage of the one polarity for aligning the liquid crystal molecules in the first direction and the voltage of the other polarity for aligning the liquid crystal molecules in the second direction is applied between the electrodes, a director is aligned in an arbitrary direction corresponding to the applied voltage and in a cone angle formed by the first direction and the second direction. The method comprises a property evaluation step of acquiring a physical property including a tilt angle defined by xc2xd of the cone angle of the liquid crystal and spontaneous polarization of the liquid crystal molecules of the liquid crystal; a computation step of acquiring a capacitance of the liquid crystal display device per unit area; and a determination step of determining whether or not the acquired tilt angle, spontaneous polarization and capacitance satisfy the following equation 2:
C(xcex82/Ps2)xe2x89xa68xe2x80x83xe2x80x83(2)
where C[F/cm2] is the capacitance of the liquid crystal display device per unit area, xcex8[o] is the tilt angle defined by xc2xd of the cone angle of the liquid crystal, and Ps[nC/cm2] is the spontaneous polarization of the liquid crystal molecules of the liquid crystal.
This method can permit the display burning of such a liquid crystal display device to be evaluated objectively by using the value of the normalized permittivity xcex5s[=xcex5(xcex82/Ps2)] as an index. The use of the normalized capacitances, can allow the degree of display burning of the liquid crystal display device to be predicted without actually measuring the display burning of the liquid crystal display device. This contributes to reducing the time and cost of producing the liquid crystal display device.